When perforating guns are fired downhole, particularly when conveyed on tubing, extreme shock loads are applied to the tubing string at the time the gun or guns are fired. These shock loads can prevent release of the guns from the tubing string and can complicate the withdrawal of the tubing string after firing. Conventional techniques for firing tubing-conveyed perforating guns involve firing the guns while attached to the tubing string. Typical of such applications are U.S. Pat. No. 5,680,905. More recently, a tool has been developed by Schlumberger which releases from the tubing string as it fires. Thus, the prevailing methods described above have a significant drawback in that the shock loads of the gun firing are transmitted to the conveying tubing string if the gun and tubing string are in any way securely attached at the time of firing.
One of the objectives of the present invention is to ensure that the gun is physically detached from the conveying tubing string at the time that it is fired. In that way, any shocks from the gun are not conveyed into the tubing. There are no issues of difficulty of removal of the tubing. The gun is simply dropped and fires as it clears the portion of the tubing string which had previously supported it. Another objective of the present invention is to initiate the firing sequence in a variety of ways, with the preferred techniques being applied pressure and dropping of a weight bar. Those and other advantages of the invention will be appreciated by those skilled in the art by reviewing the preferred embodiment described below.